Shifting between speed ratios in an automatic v transmission involves an exchange of on-coming and off-going fluid operated friction elements and is generally characterized as comprising three successive phases: a fill phase, a torque phase and an inertia phase. In the fill phase, the on-coming element is prepared for torque transmission; in the torque phase, the torque exchange occurs without a corresponding speed change; and in the inertia phase, the speed change occurs.
In shift controls based on an open-loop control philosophy, the fluid pressure supplied to the on-coming element during the torque and inertia phases is progressively increased in accordance with a predetermined pressure schedule. The pressure values are calibrated to achieve optimum shift quality in a nominal vehicle, but vehicle-to-vehicle variability and performance variations which occur over time may adversely affect the shift quality actually achieved. For this reason, adaptive control techniques have been employed to adjust the predetermined pressure schedule, based on a measure of the deviation of the actual shift quality from the desired or optimum shift quality. An example of such a control is set forth in the U.S. Pat. No. to Downs et al. 4,653,350, issued March 31, 1987, and assigned to General Motors Corporation. According to that control, the predetermined pressure schedule is adjusted in accordance with the deviation between a measure of the time interval required for the speed ratio to occur and a reference interval representative of optimum shift quality.
A complicating phenomenon experienced with the above-described control is that certain factors unrelated to pressure scheduling tend to extend the latter portion of the speed ratio progression, especially as the shift nears completion. The principle factor, especially in high torque upshifts, is the increase in input torque as the engine is decelerated from its pre-shift speed to its post-shift speed. Other factors, such as the removal of engine torque reduction controls, also extend the latter portion of the speed ratio progression. While these factors do not significantly degrade the shift quality per se, they tend to increase the time interval measured by the adaptive control, thereby resulting in unwarranted adaptive adjustment to the predetermined pressure schedule.